docs: add LDK signet config

[rolznz]

Summary by CodeRabbit

• Documentation
  • Added Signet network configuration documentation with environment variables and API endpoint settings for users to configure their Signet setup.

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📝 Walkthrough

Walkthrough

Documentation update adding a Signet configuration subsection to the LDK Network Configuration for Mutinynet, specifying three environment variables for Signet-specific mempool and Esplora server endpoints.

Changes

Cohort / File(s)	Summary
Documentation Update README.md	Added Signet configuration block with three environment variables (MEMPOOL_API, LDK_NETWORK, LDK_ESPLORA_SERVER) providing network-specific endpoints for Signet setup

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

Poem

🐰 With whiskers twitch and hoppy delight,
Signet's config now shines oh so bright!
Mempool and Esplora, all neatly arranged,
Our documentation network has beautifully changed! ✨

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately describes the main change—adding LDK Signet configuration documentation to the README.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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